1. Field of the Invention
The invention relates to the field of monitoring free surfaces, such as roadways, paths, crossings, driveways, steps etc. for the formation of snow and ice. 2. Prior Art
German Patent 2 514 489 discloses a system of this type for indicating the presence of snow and ice. Its object is to indicate and register the risk of the formation of snow and ice coatings on roadways, sidewalks, steps, bridges, ramps or roofs and optionally to prevent the formation of snow and ice coatings by switching heating installations on and off. The known device for indicating the presence of snow and ice has a sensor unit containing moisture and temperature sensors disposed in the surface to be monitored, a control device and a connecting line between the sensor unit and control device. A substantial advantage of the known device for indicating the presence of ice and snow is that a separate environmental temperature sensor is dispensed with. The temperature sensor which is in any event necessary to measure the temperature of the moisture electrodes is influenced at least indirectly by the environmental temperature. Disposed within the sensor housing is a thermostat which is controlled by the temperature sensor and which switches the heating current circuit on and off in dependence on the sensor temperature. Also disposed in the sensor housing is a voltage amplifier connected in series with the electrodes. As a result of the thermostatic control of the heating current circuit of the two resistances connected in series, a two point control of the temperatures of the moisture electrodes is produced which is sufficiently reliable and precise for most applications.
U.S. Pat. No. 3,164,820 discloses a snow and ice indicator in which the moisture electrodes are incorporated together with a thermostat and a heating element in a first block and a temperature sensor in a second block in the road surface to be monitored. The combination of the thermostat and heating element results in the moisture sensor always being maintained at a predetermined temperature of about 38.degree. F.